medRxiv preprint doi: https://doi.org/10.1101/2021.07.23.21261026; this version posted July 25, 2021. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. All rights reserved. No reuse allowed without permission. 1 Immunogenicity and safety of a third dose, and immune persistence of 2 CoronaVac vaccine in healthy adults aged 18-59 years: interim results 3 from a double-blind, randomized, placebo-controlled phase 2 clinical 4 trial 5 6 Hongxing Pan MSc1*, Qianhui Wu MPH2*, Gang Zeng Ph.D.3*, Juan Yang Ph.D.1, Deyu 7 Jiang MSc4, Xiaowei Deng MSc2, Kai Chu MSc1, Wen Zheng BSc2, Fengcai Zhu M.D.5†, 8 Hongjie Yu M.D. Ph.D.2,6,7†, Weidong Yin MBA8† 9 10 Affiliations 11 1. Vaccine Evaluation Institute, Jiangsu Provincial Center for Disease Control and 12 Prevention, Nanjing, China 13 2. School of Public Health, Fudan University, Key Laboratory of Public Health Safety, 14 Ministry of Education, Shanghai, China 15 3. Clinical Research Department, Sinovac Biotech Co., Ltd., Beijing, China 16 4. Covid-19 Vaccine Department, Sinovac Life Sciences Co., Ltd., Beijing, China 17 5. Jiangsu Provincial Center for Disease Control and Prevention, Nanjing, China 18 6. Shanghai Institute of Infectious Disease and Biosecurity, Fudan University, 19 Shanghai, China 20 7. Department of Infectious Diseases, Huashan Hospital, Fudan University, 21 Shanghai, China 22 8. Sinovac Biotech Co., Ltd., Beijing, China NOTE: This preprint reports new research that has not been certified by peer review and should not be used to guide clinical practice. medRxiv preprint doi: https://doi.org/10.1101/2021.07.23.21261026; this version posted July 25, 2021. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. All rights reserved. No reuse allowed without permission. 23 24 *: These authors contributed equally. 25 †: These authors jointly supervised this work. 26 27 Corresponding to Fengcai Zhu, Jiangsu Provincial Center for Disease Control and 28 Prevention, Nanjing 210000, China, Email: [email protected]; Hongjie Yu, School of 29 Public Health, Fudan University, Key Laboratory of Public Health Safety, Ministry of 30 Education, Shanghai 200032, China, E-mail: [email protected]; Weidong Yin, Sinovac 31 Biotech Co., Ltd., Beijing 100085, China, Email: [email protected]. 32 33 Word count (abstract): 383 34 Word count (main text): 3265 medRxiv preprint doi: https://doi.org/10.1101/2021.07.23.21261026; this version posted July 25, 2021. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. All rights reserved. No reuse allowed without permission. 35 Abstract 36 Background 37 Large-scale vaccination is being implemented globally with CoronaVac, an inactivated 38 vaccine against coronavirus disease 2019 (COVID-19). Immunogenicity and safety 39 profiles of homologous two-dose schedules have been published. We report interim 40 results of immune persistence, and the immunogenicity and safety of a third dose of 41 CoronaVac. 42 Methods 43 In this ongoing, placebo-controlled, double-blind phase 2 trial in 18-to-59-year-olds, 44 we randomly assigned subjects, 1:1:1:1, to one of four schedules to receive a third 45 dose, 28 days or 6 months after two two-dose regimens (14-day or 28-day apart): 46 schedule 1: days 0, 14, 42; schedule 2: days 0, 14, 194; schedule 3: days 0, 28, 56; 47 schedule 4: days 0, 28, 208. For each schedule, participants were randomly assigned 48 to either a medium-dose group (3 μg per 0.5 mL of aluminum hydroxide diluent per 49 dose), a high-dose group (6 μg), or a placebo group (2:2:1). The primary outcome 50 was geometric mean titers (GMTs) of neutralizing antibody to live SARS-CoV-2. 51 Results 52 Overall, 540 participants received a third dose. In the 3 μg group, neutralizing 53 antibody titers induced by the first two doses declined after 6-8 months to below the 54 seropositive cutoff (GMT: 4.1 [95%CI 3.3-5.2] for Schedule 2 and 6.7 [95%CI 5.2-8.6] 55 for Schedule 4). When a third dose was given 6-8 months after a second dose, GMTs 56 assessed 14 days later increased to 137.9 [95%CI 99.9-190.4] for Schedule 2, and 57 143.1 [95%CI 110.8-184.7] for Schedule 4, approximately 3-fold above Schedule 1 medRxiv preprint doi: https://doi.org/10.1101/2021.07.23.21261026; this version posted July 25, 2021. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. All rights reserved. No reuse allowed without permission. 58 and Schedule 3 GMTs after third doses. Similar patterns were observed for the 6 μg 59 group. The severity of solicited local and systemic adverse reactions reported within 60 28 days after the third dose were grade 1 to grade 2 in all vaccination cohorts. None 61 of the fourteen serious adverse events were considered to be related to vaccination. 62 Conclusions 63 A third dose of CoronaVac administered 6 or more months after a second dose 64 effectively recalled specific immune response to SARS-CoV-2, resulting in a 65 remarkable increase in antibody levels, and indicating that a two-dose schedule 66 generates good immune memory. Optimizing the timing of a booster dose should 67 take into account immunogenicity, vaccine efficacy/effectiveness, local epidemic 68 situation, infection risk, and vaccine supply. (Funded by the National Key Research 69 and Development Program, Beijing Science and Technology Program and National 70 Science Fund for Distinguished Young Scholars; ClinicalTrials.gov number, 71 NCT04352608.) medRxiv preprint doi: https://doi.org/10.1101/2021.07.23.21261026; this version posted July 25, 2021. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. All rights reserved. No reuse allowed without permission. 72 Introduction 73 In response to the coronavirus disease 2019 (COVID-19) pandemic, twenty COVID-19 74 vaccines have been approved for use1 and more than 3.73 billion doses have been 75 administered as of July 20, 20212. Real-world evidence has shown that vaccination 76 effectively reduces SARS-CoV-2 transmission3 and decreases COVID-19 burden of 77 disease4-6. A hope and expectation are that herd immunity can be achieved through 78 mass vaccination to curb the pandemic. Amid the ongoing effort to vaccinate target 79 populations, two critical questions are of great interest to scientists and 80 policy-makers developing strategies to stop the pandemic: how durable is 81 vaccine-induced immunity, and will people need a booster dose? And if so, when will 82 people need a booster dose? 83 84 CoronaVac (Sinovac Life Sciences, Beijing, China), an inactivated vaccine against 85 COVID-19, has been authorized for conditional use in China7 and is included in the 86 World Health Organization’s (WHO) emergency use listing8. CoronaVac has been 87 administered in 26 countries1 and is increasing the supply of COVID-19 vaccines 88 through COVAX9. In China, a total of 1.46 billion doses COVID-19 vaccines have been 89 administered as of July 1910, the vast majority of which are inactivated vaccines. 90 91 Evidence from real-world use of CoronaVac with a two-dose schedule in Chile 92 demonstrated that the vaccine effectively prevents laboratory-confirmed COVID-19, 93 and with highest effectiveness against the more severe outcomes (hospitalization, 94 ICU admission, and death)11. However, persistence of CoronaVac vaccine-induced medRxiv preprint doi: https://doi.org/10.1101/2021.07.23.21261026; this version posted July 25, 2021. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. All rights reserved. No reuse allowed without permission. 95 immunity is unknown, and the immunogenicity and safety of a booster dose has not 96 been determined. We report relevant findings from an ongoing phase 2 clinical trial 97 of CoronaVac; we compare immunogenicity and safety of a third homologous dose 98 given at an interval of 6 months or more following the second dose to an alternative 99 schedule with a dose-2 to dose-3 interval of 28 days. 100 101 Methods 102 Study design and participants 103 Our single-center, double-blind, randomized, placebo-controlled, phase 2 clinical trial 104 of CoronaVac was initiated in Jiangsu, China on May 3, 2020, with two two-dose 105 schedules in the original protocol. Healthy adults aged 18-59 years were eligible for 106 enrollment. Exclusion criteria can be found in a previous publication12. Eligible 107 participants were recruited and randomly allocated (1:1) to schedules with either a 108 14-day interval or a 28-day interval; within each schedule group, subjects were 109 randomly allocated to either a medium-dose group (3 μg per 0.5 mL of aluminum 110 hydroxide diluent per dose), a high-dose group (6 μg per 0.5 mL of aluminum 111 hydroxide diluent per dose), or a placebo group (2:2:1). Interim results on safety, 112 tolerability, and immunogenicity for these two doses have been reported12.